1. Field of the Invention
The present invention relates to a magnetic head drive device for modulating the magnetic field of a magnetic head in accordance with an information signal to be recorded, and a magnetic recording apparatus for recording information on a recording medium using the magnetic head drive device.
2. Related Background Art
As a recording method of a conventional magnetooptical recording apparatus, an optical modulation method, a magnetic field modulation method, and the like are known. In particular, the magnetic field modulation method which can directly overwrite new data on old data is advantageous in terms of the recording speed, and the like. FIG. 1 is a schematic view showing the arrangement of a magnetooptical recording apparatus adopting the magnetic field modulation method. Referring to FIG. 1, a magnetooptical disk 1 as an information recording medium has a magnetooptical recording layer 1a, and is rotated by a motor 5. A magnetic head 2 which has an excitation coil Lh wound around a magnetic core is arranged above the upper surface of the magnetooptical disk 1, and an optical head 4 is arranged below the lower surface of the disk 1 to oppose the magnetic head 2.
When an information signal is recorded, a current is supplied from a laser drive circuit 7 to a semiconductor laser 6 arranged inside the optical head 4 as a light source, and a high-power laser beam is irradiated as a very small beam spot onto the recording layer la of the rotating magnetooptical disk 1, thereby raising the temperature of the recording portion to a temperature equal to or higher than the Curie temperature of the recording layer. On the other hand, the magnetic head 2 is driven by a drive device 3 to generate a bias magnetic field which is modulated in correspondence with information to be recorded, and applies the generated magnetic field to the temperature raised portion of the recording layer 1a. Then, the direction of magnetization of the temperature raised portion on the recording layer 1a aligns in the direction of the bias magnetic field, and an information mark is recorded on the recording layer 1a. When the recorded information signal is reproduced, the laser drive circuit 7 supplies a current smaller than that in the recording mode to the semiconductor laser 6, and a low-power laser beam is irradiated as a very small beam spot onto the recording layer 1a. Then, a sensor detects rotation of the plane of polarization of reflected light due to the recorded magnetization.
Recently, in order to record information at higher density, the recording method is shifting from the mark position recording method which assigns a meaning to information at the central position of a mark to the mark edge recording method which assigns a meaning to information at the edge position of a mark. In the mark edge recording method, the edge of an information mark must be clearly recorded, and for this purpose, the reversing speed of the bias magnetic field applied from the magnetic head in the recording mode must be increased.
As a drive device for a magnetic head, which satisfies the above-mentioned requirements, a device disclosed in, e.g., Japanese Laid-Open Patent Application No. 63-94406 is known. FIG. 2 is a circuit diagram showing this drive device. Referring to FIG. 2, the excitation coil Lh generates a bias magnetic field for the magnetic head 2, and auxiliary coils L1 and L2 are used for switching the direction of the magnetic field at high speed. Switch elements S1 and S2 are used for switching the direction of a current to be supplied to the excitation coil Lh, and current sources IS1 and IS2 are respectively connected to the auxiliary coils L1 and L2. The inductances of the auxiliary coils L1 and L2 are set to be sufficiently larger than that of the excitation main coil Lh. In this drive device, the direction of a current to be supplied to the excitation main coil Lh is switched by controlling the switch elements S1 and S2 to be alternately turned on, thereby switching the polarity of the generated magnetic field in correspondence with information to be recorded.
More specifically, when the switch element S1 is ON and the switch element S2 is OFF, current paths CH1 and CH4 are rendered conductive, and current paths CH2 and CH3 indicated by broken lines are rendered nonconductive. At this time, since a current is supplied to the excitation main coil Lh upon conduction of the current path CH1, the coil Lh generates a magnetic field corresponding to the direction of the supplied current. On the other hand, when the switch element S1 is OFF and the switch element S2 is ON, the current paths CH2 and CH3 are rendered conductive, and the current paths CH1 and CH4 are rendered nonconductive. As a result, a current in a direction opposite to that described above is supplied to the excitation main coil Lh upon conduction of the current path CH2, and the coil Lh generates a magnetic field having an inverted polarity. In this case, since the inductances of the auxiliary coils L1 and L2 are sufficiently larger than that of the excitation main coil Lh, a current to be supplied maintains an almost constant value although the current paths change from CH1 to CH3 and from CH4 to CH2 before and after the ON/OFF operations of the switch elements S1 and S2. For this reason, if the ON/OFF times of the switch elements S1 and S2 are set to be sufficiently short, the direction of the current flowing through the excitation main coil Lh can be reversed within a very short period of time.
On the other hand, when the recorded information signal is reproduced, the optical head 4 irradiates a laser beam having a lower intensity than that in the recording mode onto the recording layer 1a of the rotating magnetooptical disk 1, and the information signal is reproduced on the basis of reflected light by utilizing a Kerr effect of an information mark. In this case, both the switch elements S1 and S2 of the magnetic head drive device are turned off to stop the drive operation of the main coil Lh.
However, in the conventional magnetic head drive circuit described above, when both the switch elements S1 and S2 are turned off upon completion of recording, a large counter electromotive voltage is generated in the auxiliary coils L1 and L2. For this reason, the switch elements Sl and S2 may be destroyed. The problem associated with element destruction will be described below with reference to FIGS. 3A to 3G. FIG. 3A shows a binary information signal to be recorded, and FIGS. 3B and 3C show the ON/OFF states of the switch elements S1 and S2, respectively. In FIGS. 3B and 3C, a high level corresponds to the ON state, and a low level corresponds to the OFF state. The switch elements S1 and S2 are alternately turned on/off in correspondence with an information signal, as shown in FIGS. 3B and 3C, thereby generating a magnetic field modulated in correspondence with the information signal by alternately switching the direction of the current to the main coil Lh, as shown in FIG. 3D. During reproduction, both the switch elements S1 and S2 are OFF, and the drive operation of the main coil Lh is stopped. FIG. 3G shows the power of a laser beam irradiated onto the recording layer 1a. During the recording operation, a high-power laser beam is irradiated, and the laser power is switched to low power simultaneously when the recording operation is finished and the reproducing operation is started.
FIGS. 3E and 3F show the potentials at a node (a node Q1 in FIG. 2) between the switch element S1 and the auxiliary coil L2, and a node (a node Q2) between the switch element S2 and the auxiliary coil L1. During the recording operation of information, as shown in FIGS. 3E and 3F, immediately after the switch elements S1 and S2 are turned off, voltage pulses P1 are generated by the counter electromotive forces from the auxiliary coils L2 and L1. These voltage pulses P1 are inevitably generated when the switching time of the current direction of the main coil Lh is shortened, and normally have a crest value of about several tens of volts. More specifically, during the recording operation, one of the switch elements S1 and S2 is ON, and the current paths of the auxiliary coils L1 and L2 are not rendered nonconductive. For this reason, currents of the auxiliary coils L1 and L2 do not abruptly decrease and do not generate voltage pulses exceeding several tens of volts. For this reason, when a transistor is used as the switch element, a transistor which has a rated maximum withstanding voltage of about 100 V can be selected so as not to be destroyed by the above-mentioned voltage pulses P1.
However, when the recording operation is finished and both the switch elements S1 and S2 are turned off, since the current paths of the auxiliary coils L1 and L2 are forcibly rendered nonconductive, currents of the auxiliary coils L1 and L2 abruptly decrease. For this reason, as shown in FIGS. 3E and 3F, when the switch elements S1 and S2 are turned off, voltage pulses P2 larger than the voltage pulses P1 during the recording operation are generated by the counter electromotive forces from the auxiliary coils L1 and L2. Such a voltage pulse P2 has a considerably larger crest value than that of the voltage pulse P1 during the recording operation, and sometimes reaches several hundreds of volts exceeding the rated maximum withstanding voltage of the switch element. Therefore, when a high voltage exceeding the rated maximum withstanding voltage of the switch element is applied to the switch element, the switch element may be destroyed.
In particular, in a recording medium which is formatted to have preformat regions for reproduction only, in which sector marks and address information are recorded in units of sectors, and data regions for recording data, since the recording operation is temporarily stopped on the preformat regions, if information is continuously recorded on a plurality of sectors, the above-mentioned large voltage pulse P2 is repetitively generated, and the switch elements are more easily destroyed. As described above, in recent years, the recording method is shifting from the mark position recording method to the mark edge recording method. In the mark edge recording method, it is required to shorten the current inversion time of the main coil of the magnetic head, and to shorten the reversal time of the magnetic field to be applied to the recording medium. However, since the voltage pulse P2 becomes higher as the switching time from the ON to OFF state of the switch element becomes shorter, the voltage pulse P2 becomes still higher in the magnetic head drive circuit for the mark edge recording method. In order to solve these problems, elements with a high withstanding voltage can be used. However, such a switch element with a high withstanding voltage has a large 0N resistance, and increases power consumption of the drive circuit, thus posing another problem. For this reason, the switch element with a high withstanding voltage is not an effective solution.